If you want to shoot something light, look for a clay load, like winchester AA or those federal/winchester 100-round bulk packs (if you wanna shoot that much). The winchester AA handicap or similar will be as low as you can go.
If you want something that'll rattle your teeth a little, try some 3" slugs.
What you're looking for on a box of shotshells that will help you guide your decision is the velocity and oz. of shot. The more of either, the harder it'll kick. Often this info is combined in a relative number called dram or dr. eq on the box. The higher the number, the harder it'll hit ya.
The numbers have a very narrow range. If memory serves, the dr. eq. on a box of winchester heavy field load (bird shot, #4, #2 etc) is 3 1/4, and the dr. eq on a box of winchester AA xtra-lite target load is 2 3/4 to give you an idea.
There is also Mag and Max designations for dr. eq. I assume Mag is greater than Max, but someone will correct if there is an error...